222 On the Temperature of Incandescence. 



made but little progress toward incandescence compared with 

 the other. If a number of little discs of platinum are arranged 

 in a figure within a hard glass combustion-tube, and while the 

 tube is highly heated over a blast lamp are imbedded in the 

 glass, and the tube then allowed to cool gradually in a dark 

 room, — long after the tube has become invisible the pieces of 

 platinum may be distinctly seen. 



Again, take a hard glass combustion-tube and within its mid- 

 dle third arrange a series of pieces of platinum, copper, iron, 

 lead, asbestos, j^umice, and soft and hard glass, and heat them 

 in a combustion-furnace to bright redness. Then, partly closing 

 the ends of the tube with the finger, to prevent a current of cold 

 air through it, carry it into a perfectly dark room and watch it 

 cool. The tube soon becomes absolutely invisible, but long after- 

 ward the objects within it, with the exception of the pieces of 

 hard glass, may be clearly distinguished. That the continued 

 incandescence of the contained objects within the tube is not due 

 to difference in rapidity of cooling, or other variability of condi- 

 tion, is it seems to me conclusively shown by the fact that the 

 enclosed pieces of hard glass become invisible at the same time 

 as the tube itself. 



The visibility of the heated contents of a combustion-tube in 

 the furnace, is an instance of this phenomenon, that is perfectly 

 familiar to organic analysts. 



These experiments were suggested to me by one which I tried 

 over two years ago for a gentleman who conceived the idea that 

 spirals or baskets made of threads of pure silica might be advan- 

 tageously employed instead of platinum wire spirals for suspend- 

 ing in hydrogen, water-gas, or other n on -In mi nous gas-flames, 

 to produce incandescent lights. I found that by taking two an- 

 gular fragments of amorphous quartz in separate pincers, heat- 

 ing their pointed ends in an oxy hydrogen flame, touching them 

 together where fused and suddenly drawing them apart, — 

 threads of silica as much as two decimeters long may be made. 

 No crystallized quartz I have tried will answer, because it de- 

 crepitates too violently ; and a jet of pure hydrogen and oxygen 

 seems necessary, because the common jet of coal-gas and oxygen 

 appears to produce insufficient heat to well fuse the silica. I 

 found however that when such a thread of silica was held side 

 by side with a platinum wire, of the same diameter, in the flame 



